带历史记录的Python计算器输出None的问题解决

开发了一款支持历史记录的简易Python计算器，但每次执行计算操作后，控制台都会额外输出“None”，以下是原代码及运行输出示例：

原代码 #

calculations = []
def record_history(args):
    calculations.append(args)

def history():
    if calculations == []:
        print("No past calculations to show")
    else:
        for i in calculations:
            print(i)
def add(a,b):
  return a+b
  
def subtract(a,b):
  return a-b
  
def multiply (a,b):
  return a *b

def divide(a,b):
  try:
    return a/b
  except Exception as e:
    print(e)

def power(a,b):
  return a**b
  
def remainder(a,b):
  return a % b
  
def select_op(choice):
  if choice == '#':
    return -1
  elif (choice == '$'):
    return 0
  elif (choice in ('+','-','*','/','^','%')):
    while (True):
      num1s = (input("Enter first number: "))
      print(num1s)
      if num1s.endswith('$'):
        return 0
      if num1s.endswith('#'):
        return -1
        
      try:
        num1 = float(num1s)
        break
      except:
        print("Not a valid number,please enter again")
        continue
    
    while (True):
      num2s = str(input("Enter second number: "))
      print(num2s)
      if num2s.endswith('$'):
        return 0
      if num2s.endswith('#'):
        return -1
      try:  
        num2 = float(num2s)
        break
      except:
        print("Not a valid number,please enter again")
        continue
    
    
    last_calculation = ""
    
    if choice == '+':
      result = add(num1, num2)
    elif choice == '-':
      result = subtract(num1, num2)
    elif choice == '*':
      result = multiply(num1, num2)
    elif choice == '/':
      result =  divide(num1, num2)
    elif choice == '^':
      result = power(num1, num2)
    elif choice == '%':
      result = remainder(num1, num2)
    else:
      print("Something Went Wrong")
      
    last_calculation =  "{0} {1} {2} = {3}".format(num1, choice, num2, result) 
    print((last_calculation ))
    print((record_history(last_calculation)))  # 问题所在
    
  elif choice == "?":
      history()
        
  else:
      print("Unrecognized operation")
        
while True:
  print("Select operation.")
  print("1.Add      : + ")
  print("2.Subtract : - ")
  print("3.Multiply : * ")
  print("4.Divide   : / ")
  print("5.Power    : ^ ")
  print("6.Remainder: % ")
  print("7.Terminate: # ")
  print("8.Reset    : $ ")
  print("8.History  : ? ")
  
  # take input from the user
  choice = input("Enter choice(+,-,*,/,^,%,#,$,?): ")
  print(choice)
  if(select_op(choice) == -1):
    #program ends here
    print("Done. Terminating")
    exit()

原运行输出示例 #

Select operation.
1.Add      : +
2.Subtract : -
3.Multiply : *
4.Divide   : /
5.Power    : ^
6.Remainder: %
7.Terminate: #
8.Reset    : $
8.History  : ?
Enter choice(+,-,*,/,^,%,#,$,?): ?
No past calculations to show
Select operation.
1.Add      : +
2.Subtract : -
3.Multiply : *
4.Divide   : /
5.Power    : ^
6.Remainder: %
7.Terminate: #
8.Reset    : $
8.History  : ?
Enter choice(+,-,*,/,^,%,#,$,?): -
Enter first number: 2
Enter second number: 3
2.0 - 3.0 = -1.0
None  # 多余的None输出
Select operation.
1.Add      : +
2.Subtract : -
3.Multiply : *
4.Divide   : /
5.Power    : ^
6.Remainder: %
7.Terminate: #
8.Reset    : $
8.History  : ?
Enter choice(+,-,*,/,^,%,#,$,?): ?
2.0 - 3.0 = -1.0
Select operation.
1.Add      : +
2.Subtract : -
3.Multiply : *
4.Divide   : /
5.Power    : ^
6.Remainder: %
7.Terminate: #
8.Reset    : $
8.History  : ?
Enter choice(+,-,*,/,^,%,#,$,?): #
Done. Terminating

问题原因 #

record_history函数仅执行添加记录的操作，没有定义return语句，Python中函数默认返回None。代码中用print(record_history(last_calculation))调用该函数时，会把函数的返回值（也就是None）打印出来，这就是多余输出的来源。

解决方案 #

直接调用record_history(last_calculation)，不需要用print包裹它。修改后的关键代码片段如下：

last_calculation =  "{0} {1} {2} = {3}".format(num1, choice, num2, result) 
print(last_calculation)
record_history(last_calculation)  # 移除外层的print

修改后的完整代码 #

calculations = []
def record_history(args):
    calculations.append(args)

def history():
    if calculations == []:
        print("No past calculations to show")
    else:
        for i in calculations:
            print(i)
def add(a,b):
  return a+b
  
def subtract(a,b):
  return a-b
  
def multiply (a,b):
  return a *b

def divide(a,b):
  try:
    return a/b
  except Exception as e:
    print(e)

def power(a,b):
  return a**b
  
def remainder(a,b):
  return a % b
  
def select_op(choice):
  if choice == '#':
    return -1
  elif (choice == '$'):
    return 0
  elif (choice in ('+','-','*','/','^','%')):
    while (True):
      num1s = (input("Enter first number: "))
      print(num1s)
      if num1s.endswith('$'):
        return 0
      if num1s.endswith('#'):
        return -1
        
      try:
        num1 = float(num1s)
        break
      except:
        print("Not a valid number,please enter again")
        continue
    
    while (True):
      num2s = str(input("Enter second number: "))
      print(num2s)
      if num2s.endswith('$'):
        return 0
      if num2s.endswith('#'):
        return -1
      try:  
        num2 = float(num2s)
        break
      except:
        print("Not a valid number,please enter again")
        continue
    
    
    last_calculation = ""
    
    if choice == '+':
      result = add(num1, num2)
    elif choice == '-':
      result = subtract(num1, num2)
    elif choice == '*':
      result = multiply(num1, num2)
    elif choice == '/':
      result =  divide(num1, num2)
    elif choice == '^':
      result = power(num1, num2)
    elif choice == '%':
      result = remainder(num1, num2)
    else:
      print("Something Went Wrong")
      
    last_calculation =  "{0} {1} {2} = {3}".format(num1, choice, num2, result) 
    print(last_calculation)
    record_history(last_calculation)  # 已修改：移除print
    
  elif choice == "?":
      history()
        
  else:
      print("Unrecognized operation")
        
while True:
  print("Select operation.")
  print("1.Add      : + ")
  print("2.Subtract : - ")
  print("3.Multiply : * ")
  print("4.Divide   : / ")
  print("5.Power    : ^ ")
  print("6.Remainder: % ")
  print("7.Terminate: # ")
  print("8.Reset    : $ ")
  print("8.History  : ? ")
  
  # take input from the user
  choice = input("Enter choice(+,-,*,/,^,%,#,$,?): ")
  print(choice)
  if(select_op(choice) == -1):
    #program ends here
    print("Done. Terminating")
    exit()

修改后的运行输出示例 #

Select operation.
1.Add      : +
2.Subtract : -
3.Multiply : *
4.Divide   : /
5.Power    : ^
6.Remainder: %
7.Terminate: #
8.Reset    : $
8.History  : ?
Enter choice(+,-,*,/,^,%,#,$,?): ?
No past calculations to show
Select operation.
1.Add      : +
2.Subtract : -
3.Multiply : *
4.Divide   : /
5.Power    : ^
6.Remainder: %
7.Terminate: #
8.Reset    : $
8.History  : ?
Enter choice(+,-,*,/,^,%,#,$,?): -
Enter first number: 2
Enter second number: 3
2.0 - 3.0 = -1.0  # 不再输出None
Select operation.
1.Add      : +
2.Subtract : -
3.Multiply : *
4.Divide   : /
5.Power    : ^
6.Remainder: %
7.Terminate: #
8.Reset    : $
8.History  : ?
Enter choice(+,-,*,/,^,%,#,$,?): ?
2.0 - 3.0 = -1.0
Select operation.
1.Add      : +
2.Subtract : -
3.Multiply : *
4.Divide   : /
5.Power    : ^
6.Remainder: %
7.Terminate: #
8.Reset    : $
8.History  : ?
Enter choice(+,-,*,/,^,%,#,$,?): #
Done. Terminating

内容的提问来源于stack exchange，提问作者dharsh